An ink jet printer of the type frequently referred to as "drop-on-demand" type has at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink is contained in a plurality of channels. Power pulses cause the droplets of ink to be expelled, as required, from orifices at the ends of the channels.
In a thermal ink jet printer, the power pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in that particular channel and ink bulges from the channel orifice. At that stage, the bubble begins to collapse. The ink within the channel retracts and separates from the bulging ink which forms a droplet moving in a direction away from the channel orifice and towards the recording medium. The channel is then re-filled by capillary action, which in turn draws ink from a supply container. Operation of a thermal ink jet printer is described in, for example, U.S. Pat. No. 4,849,774.
One particular form of thermal ink jet printer is described in U.S. Pat. No. 4,638,337. That printer is of the carriage type and has a plurality of printheads, each with its own ink supply cartridge, mounted on a reciprocating carriage. The channel orifices in each printhead are aligned perpendicularly to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction. The recording medium is then stepped, perpendicularly to the line of carriage movement, by a distance equal to the width of the printed swath. The carriage is then moved in the reverse direction to print another swath of information.
Typically, the recording medium is moved through the printer by a first sheet handling device using friction rollers having high surface friction characteristics relative to the friction between the sheets of the recording medium in a stack or ream. Either bottom document feeders or top document feeders are used to feed the recording medium to a printing zone characterized by traversal of the printhead therethrough during printing.
A second sheet handling device continues to move the recording medium from the printing zone into a catch tray after the first sheet handling device can no longer move the recording medium through the printing zone. The second handling device can include a drive roller and an idler roller. The drive roller has a high friction surface which contacts the bottom side or non-image side of the recording medium and provides the transporting force for the recording medium. The idler roller is rotatably mounted above the recording medium path of movement with the idler roller axis being parallel to the axis of the drive roller. The idler roller teeth are urged into contact with the surface of the drive roller to form a driving nip by either gravity or spring, so that the idler teeth apply a predetermined force that is substantially normal to the drive roller surface for driving the printed recording medium into the catch tray.
Due to a minimum carriage size necessary to carry the printheads, the recording medium is not held to the platen between where the recording medium exits the printing zone and where the recording medium reaches the second handling device. This area, where no holddown of the recording medium occurs, presents special problems, since the printhead to paper gap is small. The face of the printhead or worse, the carriage holding the printhead, can contact the recording medium if the recording medium does not lay flat in the printing area. For instance, silica coated papers, which tend to curl toward the coated side (especially in humid conditions) can contact the face of the printhead or even the carriage itself. When this occurs various problems result, such as: paper dust or silica dust which can be deposited on the face of printhead by contact with paper may contaminate the nozzles; ink can smear from the face of the printhead onto the edge of the paper; the paper can be forced out of registration when the paper "catches" on the printhead or carriage; printhead holders can be forced out of alignment; and the motion of the carriage can be impeded during the scan.
In U.S. Pat. No. 5,065,169 to Vincent et al., a device to assure paper flatness and pen-to-paper spacing during printing is described. A carriage mounted to carry an ink jet pen across a sheet for printing includes a skidlike spacer to ride upon the printed surface. The spacer maintains a preselected spacing between the pen and the printed surface and, also maintains paper flatness at the localized area of printing.
U.S. Pat. No. 5,163,674 to Parks describes transportation of a recording medium with wet images without degradation of print quality by a drive means comprising a drive roller and a toothed idler roller. The toothed idler roller has spaced teeth with rounded distal ends which extend from a hub to form a nip with the drive roller through which the recording medium is transported.
U.S. Pat. No. 5,265,856 describes a sheet feeder for a copying machine. The sheet feeder, such as a recirculating document handler for a document machine, includes movable sheet flattening guide arms for flattening out upturned sheet corner dog-ears in the leading edges of a document sheet as each sheet is fed from the stacking tray of the document handler.